The invention relates to an apparatus and method for precision overboring an existing bore through a workpiece and, more particularly, relates to such a method and apparatus for forming a close tolerance bore through a metal shaft having an existing bore of variable diameter extending longitudinally through it in an arcuate path caused by bending or sagging of the workpiece due to its weight and supporting arrangement.
In the operation and maintenance of large rotating machinery such as turbines and dynamoelectric machines it is often necessary or desirable to overbore an existing passageway through the drive shaft or supporting shaft of such machines for a variety of reasons. Due to the enormous dynamic stresses placed on such shafts, it is necessary to maintain the dimensions of any such overbore within very close tolerances in order to avoid the introduction of an undesirable imbalance in the shaft.
Of course, various techniques of overboring existing bores through relatively small workpieces are well-known and widely used in situations either where close tolerances need not be maintained, or in circumstances such that the pre-existing bore has a uniform diameter over its length and is substantially straight. Typical examples of suitable tools for effecting such relatively simple overboring operations are shown, e.g., in the U.S. Pat. Nos. 967,789, which issued on Aug. 16, 1910, and 3,028,772, which issued on Apr. 10, 1962. As is common with the type of overboring tools shown in those two patents, they each utilize a boring tool pilot mounted ahead of the tool bit in order to guide the bit along a cutting path established by an existing bore through the workpiece.
Such prior art tools are acceptable for overboring a relatively short hole along a straight, uniform diameter existing bore in an application that requires relatively little metal to be removed by the overboring operation. However, such tools are not suitable for forming a relatively large overbore through a long, heavy machine shaft of the type described at the outset, above. Accordingly, prior to the present invention, it was necessary to remove such a large shaft from operation and transport it to a work site where heavy lathes were available to rotate the shaft relative to a conventional single blade bit, whenever it was desired to perform a close-tolerance overboring operation on it. In such overboring operations on large shafts it frequently is desirable to enlarge an existing bore by between 4 and 5 inches in diameter. Thus, it will be understood that if such an overboring operation is to be accomplished by making a single pass of an overboring tool through the shaft, an apparatus and method must be employed that will accommodate the extreme forces and temperatures developed by the removal of such a volume of metal, without causing the resultant bore diameter to waiver beyond established close tolerances that might cause the shaft to be unbalanced and ruined.
By performing such overboring operations on large shafts in a major workshop that has a lathe big enough to rotate the shaft during the overboring operation, it is possible to form a close tolerance overbore through the shaft by making repeated boring passes with a single cutting edge boring tool that is rotated in a direction counter to the direction of rotation of the shaft on the lathe. Such rotation of the shaft during that type of conventional prior art overboring operation served to effectively counteract the inevitable sag in the shaft caused by its weight and usual supporting arrangement. The major disadvantages inherent in such earlier state-of-the-art overboring methods as applied to large machine shafts, are that the shaft must be removed from operation, then transported long distances (in many cases), in order to take it to a workshop that is suitable for performing the overboring operation.